The fertility rate (mean number of children per adult woman) varies in European countries between a low of 1.2 (Poland and Portugal) and a high of 1.9 (Ireland and France). For each woman, the number of children is a whole number, such as 0 or 1 or 2. Explain why it makes sense to measure a mean number of children per adult woman (which is not a whole number), for example, to compare these rates among European countries or with Canada (1.6), the United States (1.9), and Mexico (2.2).